1. Field of the Invention
The present invention relates to a dispersion-compensating module which improves the transmission quality of large-capacity, high-speed optical transmission systems of WDM (Wavelength Division Multiplexing) type.
2. Related Background Art
A WDM type optical transmission system is a system which transmits a plurality of signal light components within a 1.55-μm wavelength band (1.53 μm to 1.57 μm) by way of an optical fiber transmission line network, and enables large-capacity, high-speed optical communications. This optical transmission system comprises an optical amplifier for collectively optically amplifying a plurality of signal light components, and the like, in addition to an optical fiber line which is a transmission medium. In such WDM communications, various studies and developments are under way in order to enable communications with further larger capacity and higher speed.
One of important subjects to be studied concerning the optical transmission line is reduction of dispersion in its signal wavelength band. Namely, each signal light component has a certain bandwidth even though it is monochromatic. As a result, when dispersion occurs in the wavelength band of the signal light in the optical transmission line, the signal light having reached a receiving station by way of the optical transmission line after being sent out from a transmitting station deforms its waveform, thereby deteriorating its reception. Therefore, in the signal light wavelength band, it is desirable that the dispersion of the optical transmission line be as small as possible.
However, standard single-mode optical fibers (hereinafter referred to as SMF), having a zero-dispersion wavelength in a 1.3-μm wavelength band, already installed as an optical transmission line have a dispersion of about 16 ps/nm/km in a wavelength band of 1.53 μm to 1.57 μm which is used in WDM communications. Many of already installed optical transmission lines are constituted by such an SMF. Therefore, a dispersion-compensating module is disposed within a repeater in order to compensate for the dispersion of the optical transmission line, while making use of such an already installed optical transmission line.
With respect to the dispersion over the whole length of the optical transmission line to be compensated for, the dispersion-compensating module generates a dispersion having an opposite polarity with substantially the same absolute value. Specifically, the dispersion-compensating module comprises a dispersion-compensating optical fiber having a dispersion with a polarity opposite to that of the dispersion of the optical transmission line (including SMF), and compensates for the dispersion of the optical transmission line (including SMF) by adjusting the dispersion-compensating optical fiber to an appropriate length. Also, for reducing the size of the dispersion-compensating module, it is a common practice to wind the dispersion-compensating optical fiber into a coil having a small diameter.